Multi-antenna (MIMO: Multiple Input Multiple Output) wireless telecommunications technology is a technology of achieving a spatial multiplexing effect and a space diversity effect by providing a plurality of antennas at each of a transmitting terminal and a receiving terminal so as to utilize spatial resources during a wireless telecommunication. Studies have shown that according to a theory of information, a capacity of a MIMO system increases linearly with an increase in a smaller one of (i) the number of transmit antennas and (ii) the number of receive antennas. FIG. 1 is a diagram schematically illustrating a MIMO system. As illustrated in FIG. 1, multi-antenna wireless channels formed by multiple antennas at each of the transmitting terminal and the receiving terminal include spatial information. Precoding technology is a major technology for improving a data transmission rate with use of current spatial information, and is also a technology which uses a channel state information pretreatment transmission signal. A precoder is substantially a multi-mode beam generator, and matches transmission signals with the channels for the transmitting terminal and the receiving terminal. A basic principle of precoding is that a precoder separates transmission signals into multiple layers and causes the multiple layers to be orthogonal to one another so that (i) transmission signals in each layer can obtain a large gain after passing through channels and (ii) independent orthogonality is ensured. There are at a maximum M data layers which are transmitted between the transmitting terminal and the receiving terminal and which are orthogonal to and independent of one another (where M is the smaller one of the respective numbers of antennas at the two terminals). Orthogonal frequency division multiplexing (OFDM) technology characteristically has a great anti-fading ability and a high efficiency in frequency use, and is thus used in a high-speed data communication in a multipath environment and a fading environment. MIMO-OFDM technology, which is a combination of MIMO technology and OFDM technology, is recognized as a core technology for next-generation mobile communications systems.
For example, the 3GPP (3rd Generation Partnership Project) organization, which is an international organization in a field of mobile communications technology, has been playing an important role in standardizing the 3G (3rd generation) cellular system. The 3GPP organization has been performing a project since a second half of 2004 to design EUTRA (Evolved Universal Terrestrial Radio Access) and EUTRAN (Evolved Universal Terrestrial Radio Access Network). This project is commonly called Long Term Evolution (LTE). MIMO-OFDM technology is used for a downlink communication in an LTE system. The 3GPP organization held a conference in Shenzhen, China, in April 2008 to discuss standardization of the 4G (4th Generation) cellular system. At this conference, a concept of “Coordinated multi-point (CoMP) transmission/reception” widely attracted attention and gained support. A core idea of the concept is that a plurality of base stations simultaneously provide a communication service to a single user or multiple users so as to improve a rate of transmitting data to a user on a cell border. To realize this plan, it is essential to use a precoding method for a cooperative communication between multiple multi-antenna base stations.
Three conventional techniques below have each been known as a method of a cooperative communication between multiple multi-antenna base stations for a downlink communication in a cellular system.
(1) Method of independently precoding transmission signals at a single base station and transmitting the transmission signals to a user device without modification: A serving base station and cooperative communication base stations perform precoding by a method involving a dispersion formula. Specifically, the serving base station and the cooperative communication base stations each have a precoding matrix which matches only a matrix for channels extending from the base station to a user device. Precoded signals are transmitted without modification from the serving base station and the cooperative communication base stations to the user device. The user device receives signals which have been (i) precoded by the method involving a dispersion formula, (ii) passed through their respective channels, and (iii) additively and directly combined with one another. The received signals can be expressed by the following numerical formula:y=(√{square root over (P1)}H1W1+√{square root over (P2)}H1W2+ . . . +√{square root over (PN)}HNWN)x+n     y: received signal    x: transmission data    n: noise    N: total number of a serving base station and cooperative base stations    √{square root over (P1)}, √{square root over (P2)}, . . . , √{square root over (PN)}: transmission electric power coefficients    H1, H2, . . . , HN: matrices for channels extending from a serving base station and cooperative base stations to a user device    W1, W2, . . . , WN: precoding matrices of a serving base station and cooperative base stations
A core idea of this method (1) is that W1, W2, . . . , WN match H1, H2, . . . , HN, respectively. FIG. 2 illustrates three base stations which employ this precoding method for a cooperative communication. This method is advantageous in that it can (i) be performed easily and flexibly and (ii) produce a good performance (see Non Patent Literature 1).
(2) Method of independently precoding transmission signals at a single base station, weighting a precoding matrix, and transmitting the transmission signals to a user device: To improve the performance produced by the method (1), this method (i) creates various weight values, that is, weight coefficients, (ii) weights a result of independent precoding at each base station with the weight coefficients, and (iii) transmits the weighted result to a receiving terminal. This method thus allows the receiving terminal to obtain a gain of coherent composition. The user device receives signals which have been (i) precoded by the method involving a dispersion formula, (ii) passed through their respective channels, and (iii) weighted, so that weighted results are combined with one another. The received signals can be expressed by the following numerical formula:y(√{square root over (P1)}H1W1d1√{square root over (P2)}H2W2d2+ . . . +√{square root over (PN)}HNWNdN)x+n     y: received signal    x: transmission data    n: noise    N: total number of a serving base station and cooperative base stations    √{square root over (P1)}, √{square root over (P2)}, . . . , √{square root over (PN)}: transmission electric power coefficients    H1, H2, . . . , HN: matrices for channels extending from a serving base station and cooperative base stations to a user device    W1, W2, . . . , WN: precoding matrices of a serving base station and cooperative base stations    d1, d2, . . . , dN: weight coefficients (which are each normally a value expressed as a complex number)
A core idea of this method (2) is that (i) W1, W2, . . . , WN match H1, H2, . . . , HN, respectively, and (ii) each base station adjusts a phase of channel matching result so that the user device can superpose identical phases as much as possible for a gain of coherent composition. FIG. 3 illustrates three base stations which employ this cooperative precoding method. This method is advantageous in that it can produce a good performance (see Non Patent Literature 2).
(3) Method of an identically precoding transmission signals at a plurality of base stations and transmitting the transmission signals to a user device without modification: Complexity of identical precoding linearly increases with an increase in the total number of a serving base station and cooperative communication base stations. To reduce the complexity, a plurality of base stations employ an identical precoding matrix. The plurality of base stations can thus (i) match the precoding matrix with a matrix for channels extending from a serving base station and cooperative communication base stations to the user device, and then (ii) transmit the transmission signals to the user device without modification. From a standpoint of the user device, matrices for channels extending from a serving base station and cooperative communication base stations to the user device is directly combined first to provide equivalent virtual channels, and the matrices of channels are matched with an identical precoding matrix. The received signals can be expressed by the following numerical formula:y=(√{square root over (P1)}H1+√{square root over (P2)}H2+ . . . +√{square root over (PN)}HN)Wx+n     y: received signal    x: transmission data    n: noise    N: total number of a serving base station and cooperative base stations    √{square root over (P1)}, √{square root over (P2)}, . . . , √{square root over (PN)}: transmission electric power coefficients    H1, H2, . . . , HN: matrices for channels extending from a serving base station and cooperative base stations to a user device    W: identical precoding matrix employed by a serving base station and cooperative base stations
FIG. 4 illustrates three base stations which employ this cooperative precoding method. This method is advantageous in that it can (i) be performed easily and (ii) reduce an amount of feedback information (see Non Patent Literature 2).